Liquid crystals (LCs) are molecules which exhibit self organization into orientationally ordered phases (nematic phases). In addition, many liquid crystals form smectic phases in which the molecules are oriented and arranged in layers. One class of smectic phase, the chiral smectic-C (S.sub.C *) liquid crystal, contains molecules oriented with their axes tilted with respect to the normal of the layers. The tilt of the axis in a given layer rotates progressively a small amount from layer to layer in the S.sub.C * liquid crystal. The total thickness of the liquid crystal layers required for the molecular axes to precess through 360.degree. is known as the supramolecular pitch. The supermolecular pitch is small, generally on the order of 1-3 microns.
Each layer of an S.sub.C * liquid crystal is spontaneously polarized and undergoes a change in orientation upon application of a threshold electrical field strength. The direction of polarization changes with each layer due to the axes tilt from one layer to the next. In order for such a phenomenon to be useful in display applications, it is necessary to "unwind" the supermolecular pitch of the ferroelectric liquid crystal, as these materials as known, so that the axes of each successive LC layer are identically oriented (the so-called "bookshelf" arrangement). FIG. 1 illustrates the supramolecular pitch of a smectic-C phase and its unwinding to obtain an aligned liquid crystal.
Ferroelectric liquid crystals (FLCs) have attracted great interest since the discovery in 1980 of electro-optic switching using surface stabilized FLCs (SSFLCs). In this process, the top and bottom surfaces of the FLC display panel (thin layer of FLC between glass electrodes) are rubbed, which causes the FLC tilt angles at these surfaces to align resulting in an "unwinding" of the helical pitch. The unwinding of the supramolecular pitch within a small gap on the order of 1-3 microns is difficult to precisely control. Further, this thin gap limits the panel size and the use of glass substrates and a low molar mass FLC results in poor mechanical properties.
As a result of the shortcomings of low molecular weight liquid crystal systems, polymeric FLCs have been investigated. Block copolymers and liquid crystals are both known to form ordered structures at the monomer, mesogenic and microdomain dimensions. These materials permit the manipulation of the liquid crystal order by control of both the liquid crystal and block copolymer components of the composition. As an added advantage, polymeric materials tend to be easier to process and provide the possibility of preparing flexible display panels. However, the higher molecular weight of the polymeric systems also lead to higher viscosity, resulting in slower response times in an applied electric field.
Liquid crystal-containing block copolymers have been reported. Scherowsky et al. (Liq. Cryst. 5:1289 (1989)) have reported a FLC (poly)acrylate which has a switching time of 0.5-5 milliseconds with a number average molecular weight (M.sub.n) of 15,000 at .about.120-130.degree. C. Takahashi et al. (Liq. Cryst. 8:33 (1991)) report that FLCs with a poly(siloxane) backbone possess a 33 millisecond switching time at 43.degree. C. Zentel and Brehmer (Macromol. Rapid Commun. 16:659 (1995)) prepared FLC elastomers by photo-crosslinking monodomain FLC poly(siloxane)s in a bookshelf arrangement. Although these results suggest promising properties using polymeric FLCs, the switching properties are too slow for use in optical display panels.
Omenat et al. (Macromol. 29:6730 (1996)) report a series of side-group FLC-diblock copolymers in which only monostable switching was observed. The composition consisted of a poly(isobutyl vinyl ether) chain and a chiral liquid crystalline block.
There remains a need to provide a FLC system which is capable of rapid bistable switching in an applied field.
It is an object of the present invention to provide a flexible, thin FLC-polymer display device which demonstrates bistable switching.
It is a further object of the present invention to provide an FLC-polymer which demonstrates rapid switching over reasonable temperature ranges.